1. Field of the Invention
The present invention relates to tensionable cable mine roof bolts, in particular, a tensionable cable mine roof bolt having a tension indicator which is adapted to be grouted and mechanically anchored in a mine roof bore hole.
2. Prior Art
Cable mine roof bolts are gaining popularity in the mining industry for their ease of handling and installation. Cable bolts are substantially easier to fit into a bore hole than the elongated rods of conventional rod bolt systems. Regardless of the height limitations in a mine, cable bolts may be adapted to bore holes of any length due to their flexibility. The strength capacity of cables exceeds that of conventional rod bolts and, therefore, cable is the preferred reinforcement for certain roof conditions.
Conventional cable mine roof bolts are installed by placing a resin cartridge including catalyst and adhesive material into the blind end of a bore hole, inserting the cable bolt into the bore hole so that the upper end of the cable bolt rips open the resin cartridge and the resin flows in the annulus between the bore hole and the cable bolt, rotating the cable bolt to mix the resin catalyst and adhesive and allowing the resin to set about the cable bolt. Typically, the resin is set after two to three minutes. Cable bolts have heretofore been primarily used as secondary roof support structures with tensionable rock bolts serving as the primary anchorage mechanism.
Tensionable cable bolts are the subject of U.S. Pat. No. 5,378,087 to Locotos and U.S. Pat. No. 5,525,013 to Seegmiller et al. Each of the bolts described therein are resin grouted at the blind end of a bore hole and following setting of the resin, they are tensioned by rotation of a nut on an externally threaded sleeve surrounding the free end of the cable. U.S. Pat. No. 5,531,545 to Seegmiller et al. and U.S. Pat. No. 5,556,233 to Kovago both disclose tensionable bolts with a mechanical anchor mounted on the upper end of the cable bolt and tensioning mechanisms disposed on their free ends for post-installation tensioning. Although these prior art cable bolts are tensionable, they require two installation steps; namely, a first step to anchor the upper end of the cable bolt in the bore hole and a second step to tension the bolt.
U.S. Pat. No. 5,375,946 to Locotos discloses a cable bolt having a shaft connected at its upper end, the shaft bearing an expansion anchor. Anchorage of the bolt occurs primarily at the upper end of the bolt by action of the expansion anchor and resin. Thus, anchorage occurs only at the blind end of the bore hole in the vicinity of the expansion anchor and the resin. Another drawback to the bolt is that it is difficult to determine the amount of tension exerted upon the bolt during installation.
It is an object of the present invention to provide a tensionable cable bolt having a plurality of locations of anchorage within a bore hole and which is tensionable to a predetermined load.